1. Field of the Invention
The present invention relates to closed loop systems having a regulator for control of a process, and more particularly to an apparatus for estimating the process frequency response and using the estimated frequency response to design the regulator.
2. Brief Description of the Prior Art
Controllers for tuning the response of a system or a process have been the subject of attention in the prior art. A typical block diagram of such a controller and process is shown in FIG. 1 wherein the output y of a process 10 is fed back through a feedback network 12, the output yxe2x80x2 of which is subtracted from a reference signal r by a comparitor 14 which outputs an error signal e. A control circuit 16 is designed to respond to the error signal e by outputting a process control signal u for directing the process 10 to output a corrected y such that the error e is zero.
Achieving automatic tuning involves identification of the process 10, and the design of the controller 16. The control output u as a function of time is applied to the process 10 to assure the desired output y. It is generally convenient in analysis of such processes and control circuitry to mathematically transform the parameters from the time domain to a frequency domain. This process is done in electrical circuit analysis by means of the well known Fourier Transform. Manipulation of design parameters is much more conveniently achieved in a frequency domain. The frequency range of interest for such applications is usually from zero up to the process critical frequency xcfx89c i.e. the frequency at which the phase of a process frequency response crosses xe2x88x92180xc2x0. This critical frequency and the process frequency response at this point are useful for control analysis and design. For traditional frequency response identification, the frequencies of the exciting signals u fed into the process/system 10 should be carefully selected based on the process bandwidth. The use of relay feedback can automatically excite an unknown stable process around this frequency xcfx89c. Such a method is described in U.S. Pat. No. 4,549,123 by Hagglund and Astrom. Furthermore, traditional frequency response identification usually involves an open loop test, while the use of relay feedback involves a closed-loop controller. A closed-loop test is preferred to an open-loop test in control applications, since it keeps the process close to the set point so that the process operates in a linear region where the frequency response is of interest.
Relay based process identification and control tuning have received a great deal of attention. The method of Astrom and Hagglund is useful in many process control applications, but it also faces two major problems. First, due to the use of a describing function approximation, the estimation of the critical point is not accurate enough for some kinds of processes. Second, the method of Astrom and Hagglund depends on the single critical point xcfx89c, and only crude controller settings can be obtained based on this single point. Attempts have been made to solve this problem, including several modified identification methods using relay-based feedback control systems in order to identify more than one point on the process frequency response. In order to accomplish this, additional linear components (or varying hysteresis width) have been connected into the system requiring additional relay tests to be performed. These methods are time consuming and the resultant estimations are still approximate in nature since they rely on repeated use of the standard method as described in Astrom and Hagglund.
It is therefore clear that a method is needed whereby more points on the process frequency response can be accurately identified from a single relay test.
It is therefore an object of the present invention to provide an improved apparatus for process frequency response estimation and controller tuning.
It is a further object of the present invention to provide an apparatus for process frequency response estimation and controller tuning for accurately identifying multiple points on the process frequency response with a single relay test.
It is a still further object of the present invention to provide an apparatus for process frequency response estimation and controller tuning for accurately identifying multiple points on the process frequency response with a single relay test, whereby the controller regulator is tuned with respect to the identified points on the process frequency response, whereby the controller system achieves an improved, uniform response.
Briefly, a preferred embodiment of the present invention includes an apparatus for tuning a regulator in a process controller. The digitized output from a standard relay and a parasitic relay are fed through a digital to analog converter providing signals at 0.5 xcfx89c, xcfx89c and 1.5 xcfx89c, to the process, where xcfx89c is the process critical frequency. The apparatus records the digitized relay output uxe2x80x2 and the digitized process output yxe2x80x2 until stationary oscillations are reached. The outputs uxe2x80x2 and yxe2x80x2 are then used to calculate the process frequency response at 0.5 xcfx89c, xcfx89c and 1.5 xcfx89c. This data is then used to design the regulator, the output of which is converted to analog form and inputted to control the process.
An advantage of the auto-tuner of the present invention is that it can estimate multiple points on a process frequency response simultaneously with one single relay test, resulting in time savings.
A further advantage of the present invention is that it gives accurate results since no approximations are made, and the computations involved are simple so that it can be easily implemented on microprocessors.
A still further advantage of the present invention is that the method is insensitive to noise and step-like load disturbances, and non-zero initial conditions.
Another advantage of the present invention is that the controller tuning methods employed work well for the general class of linear processes with different dynamics, achieving consistent satisfactory responses.